The compound semiconductors composed primarily of GaN have been used extensively for fabrication of light-emitting devices such as diodes that are capable of glowing in blue. Examples of such compound semiconductors include, in addition to GaN itself, gallium aluminum nitride (GaAlN), indium gallium nitride (InGaN), and indium gallium aluminum nitride (InGaAlN).
A typical prior art light-emitting device of the kind under consideration comprises a baseplate of electrically insulating material such as sapphire, a buffer layer overlying the baseplate and composed for example of GaxAl1−xN, where x is greater than zero and not greater than one (as taught by Japanese Unexamined Patent Publication No. 4-297023), an n-type semiconductor region of GaN or other compound semiconductor composed principally of GaN and grown epitaxially on the buffer layer, an active layer of another compound semiconductor composed principally of GaN (e.g. InGaN) and grown epitaxially on the n-type semiconductor region, and a p-type semiconductor region grown epitaxially on the active layer. The n-type semiconductor region is connected to a cathode, and the p-type semiconductor region to an anode.
The common practice in the manufacture of light-emitting devices is first to form wafers on which there are fabricated matrices of desired devices, and to cut them into the individual devices as by dicing, scribing, or cleavaging. The noted sapphire baseplate of the light-emitting devices has been a cause of trouble in such dicing of the wafers because of its extreme hardness. Sapphire itself is expensive, moreover, adding much to the manufacturing costs of the light-emitting devices.
There have been additional difficulties in connection with the sapphire baseplate. Being electrically insulating, the sapphire baseplate makes it impossible to form a cathode thereon. This inconvenience was conventionally circumvented by exposing part of the n-type semiconductor region through the active layer and p-type semiconductor region for connection to a cathode. The results were a greater surface area of the semiconductor and a corresponding increase in the costs of the light-emitting devices.
A further inconvenience arose from the fact that current flows through the n-type semiconductor region not only vertically (normal to the plane of the sapphire baseplate) but horizontally (parallel to the sapphire baseplate plane). The dimension of the n-type semiconductor region for the horizontal current flow is as small as four to five micrometers, so that the resistance of the horizontal current path of the n-type semiconductor region was very high, adding substantively to the current and voltage requirements of the prior art devices.
A still further inconvenience concerns the etching-away of parts of the active layer and p-type semiconductor region in order to expose part of the n-type semiconductor region for connection to the cathode. The n-type semiconductor region had to be dimensioned sufficiently large to allow for some errors in etching, necessitating a correspondingly elongated period of time for it to be grown epitaxially.
It has been suggested to use a conductive baseplate of silicon carbide (SiC) in substitution for the sapphire. Permitting a cathode to be formed thereon, the SiC baseplate offers such advantages over the sapphire baseplate as a smaller surface area and easier separation of the wafer by cleavaging. Offsetting these advantages, however, is the fact that SiC is even more expensive than sapphire. Another shortcoming is the difficulty of placing the n-type semiconductor region in low-resistance contact with the SiC baseplate, so that the current and voltage requirements of the light-emitting device incorporating the SiC baseplate were just as high as those of the device with the sapphire baseplate.
The present invention aims at the provision of a light-emitting device, and a method of fabrication thereof, such that the device is efficiently manufacturable at a lower cost than heretofore and is improved in performance too.